230 PHYLUM ANNELIDA. 
each side fuse above and below the gut to form the dorsal and ventral 
supporting mesenteries of the gut. The head region is at first dispro- 
portionately large, but later, by an independent process of growth, 
becomes reduced. The larva abandons its pelagic life, and becomes 
adult. 
Comparing the development of Polycheta with this, we find that the 
Trochosphere is often modified, and that segmentation tends constantly 
to appear at an earlier stage. As a further step in the same direction, 
we may note that in some Polychzeta the Trochosphere stage is no 
longer recognisable as such. 
GENERAL SURVEY OF THE CLASS CHETOPODA 
I. Oligocheeta.—The general characters may be gathered from 
the description of the earthworm, but it is to be noticed that the earth- 
worms are specialised forms, and that the fresh-water Oligochetes are 
of much simpler structure. The most essential distinction from the 
Polychzeta is to be found in the complex reproductive organs. The 
absence of gills, though general, is not universal, for a few fresh-water 
forms, such as Dero and Branchzura, possess gills of simple structure, 
while the West African Alma mnzlotica has more complex branched 
retractile gills. Among other characters may be noticed the tendency 
to variation in the structure of the excretory system. In all, with the 
exception of olosoma, certain of the nephridia are modified to serve 
as genital ducts, while in the Megascolicide the nephridia tend to be 
reduced to a mass of minute tubules ramifying over the inner surface 
of the body wall. In general the Oligochztes, however, show more 
uniformity of structure than their marine allies. 
They may be divided into two main groups—(1) the Microdrili, 
and (2) the Megadrili. The first group includes the small aquatic 
forms; of these most familiar are Tudbéfex rivulorum, often found 
in the mud of brooks, and the species of Mazs, remarkable for their 
power of asexual budding. Some Microdrili live between tide-marks. 
The leech-like Branchzobdella, which is parasitic on the gills of the 
fresh-water crayfish, is a somewhat aberrant member of the group. 
The Megadrili include the larger Oligochzetes, mostly living in earth, 
and commonly designated as “earthworms.” The largest form is a 
Tasmanian species (M@egascolides gippslandicus), measuring about 6 ft. 
in length. 
II. *Dalyohets,— As contrasted with the more or less subterranean 
earth- and mud-worms, the marine Polychzta have a richer develop- 
ment of external structures and a more complex life history. The 
external appearance is greatly modified by the relative degree of 
development of the parapodia, which are lateral outgrowths typically 
functioning as walking ‘‘legs,” or as swimming organs. A para- 
podium, when fully developed, is divisible into a ventral neuropodium 
and a dorsal notopodium. Each of these is bilobed, bears a tactile 
process or cirrus, and is fringed with firm bristles or setae, Within the 
substance of each lobe is embedded a stout needle-shaped ‘‘ aciculum,” 
